In recent years, there have been advances in the technology for solid-state imaging elements that include not only CCDs but also CMOS elements. For solid-state imaging elements in digital still cameras, these CCD elements and CMOS elements are used together. Since CMOS elements generally have a low light-sensitivity, it is preferable for the imaging lenses used with these elements to be bright lenses (lenses with small F numbers) in addition to being high-performance, compact, thin, and provided at a low cost.
In these solid-state imaging elements, microlenses are disposed on the surface thereof in order to allow efficient entry of light. Therefore, if the angle of the light entering into the solid-state imaging element is too large, “vignetting” takes place and the light does not enter into the solid-state imaging element. Therefore, it is preferable to use a telecentric optical system with a small angle of emergence and with the exit pupil of the imaging lens being as far away as possible from the imaging surface.
With the significant advances in solid-state imaging elements, it is now known that “vignetting” can be eliminated if the imaging lens has an angle of emergence in a range of approximately 20 degrees to 24 degrees.
Also, the back focus must be adequately long because of the need to place a low-pass filter for preventing color moires or an infrared cut filter for correcting the spectral sensitivity of the solid-state imaging element between the imaging lens and the solid-state imaging element.
In the imaging lens described in Japanese Laid-Open Patent Publication Number Hei 09-133860, the aperture stop is disposed closest to the imaging surface, thus shortening the exit pupil position. This makes it difficult to reduce the angle of emergence and to keep the total length of the lens system short.
Also, in the imaging lens described in Japanese Laid-Open Patent Publication Number Hei 05-188284, the aperture stop is disposed furthest toward the object side, but since there is no inflection point on the aspherical surface of the third lens or in the lens system, it is difficult to keep the angle of emergence small and to keep the total length of the lens system short.